280 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
of older rock occur at a slightly higher levei than that of the living coral, no 
fragment now rises above the mean sea-level, even on the inner protected 
reefs or near the present shore. The inference is that the patches of coral in 
the sand of the plain were protected from erosion by the sea soon after 
their final elevation, while others only a little farther seawards were exposed 
so long as to be eroded to present sea-level. This, however, does not justify 
any belief in a slowing of the formation of the plain in recent times, as these 
patches are on the landward ends of reefs the shallow flats of which were 
soon covered by sand and form the long promontories which divide the 
lagoons from one another. Indeed these, like other features of the plain— 
e. g. the old beach lines,—indicate that its formation has been almost perfectly 
regular, and probably began after the reefs had reached their present 
condition. Without the shelter of the reefs the constitution of the sand 
described above could not have been as it is. 
Dongonab Harbour Reefs.—These reefs are somewhat complicated, and only 
after having lived at Dongonab for some mouths was I able to pass the 
harbour entrances without the assistance of a native. But once understood, 
the plan of the reefs off Dongonab Harbour is very simple. They lie in three 
lines, the first being a nearly continuous surface-reef, the next a line of shoals 
with heads and patches of coral reaching to the surface, the third a series of 
rather deeper shoals with two fathoms of water generally, with rare heads 
reaching to a fathom of the surface. 
A good deal of mud and grey sand has collected about these reefs, so that 
when I saw them in 1905 I thought it possible that they were a series of 
banks of sediment with coral caps, a possible method of reef formation 
suggested by Darwin, and one which is the most probable explanation of the 
Shubuk reef labyrinth. There are, however, areas of old rock a few inches 
above the level at which growing coral covers the reefs, and this fact indicates 
that this series, like all the other reefs of the coast, owes its formation to 
faulting, three troughs separating the ridges which are now reefs. The fact 
that the outer ridges have not become surface-reefs is particularly interesting 
in connection with the ecology of coral, and will be referred to later. 
Jedda.—It would hardly be worth while considering the formation of the 
harbour of a totally insignificant place like the village of Dongonab were it not 
that it is a miniature of the harbour of Jedda, just opposite on the Arabian 
Coast. Though quite unvisited by the ordinary traveller, this, the gateway of 
the Moslem Holy Land, is crowded with ships of all sorts, from large steamers 
to Arab sambtiks, during the season of the Mecca pilgrimage. Thevery peculiar 
arrangements of the reefs which give it protection from the sea is evident by 
a glance at the plan on PI. 381. 
